(a) Field of the invention:
The present invention relates to a photographic zoom lens system and more specifically to a zoom lens system suited for use with photographic cameras using electronic image pickup tubes and solid-state image pickup devices as the image sensors.
(b) Description of the prior art:
The lens systems intended for use with photographic cameras using the electronic image pickup tubes and solid-state image pickup devices as the image sensors are designed mostly as zoom lens systems. Most of these zoom lens systems have aperture ratios of at least F/2.8 and zooming ratios of at least 3.
These zoom lens systems have the high aperture ratios and high vari-focal ratios, but comprise lens elements in a number as large as 13 to 15 and require high manufacturing costs.
As zoom lens systems of this type, lens systems have recently been disclosed by Japanese Unexamined Published Patents Application No. 123817/60 and No. 247318/62. Each of these zoom lens systems comprises 12 to 18 lens elements. Out of these lens systems, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 247318/62 has a fundamental composition consisting, in the order from the object side, a first lens unit which has positive refractive power and is held stationary for varing focal length but movable for focusing, a second lens unit which has negative refractive power and is movable for varying focal length, a third lens unit which has positive refractive power and is moved along a locus convex on the image side for correcting positional deviation of the image surface caused by variation of focal length, and a fourth lens unit which has an imaging function and is always kept fixed. This conventional example consists of 11 lens elements in total. Speaking concretely, it comprises three lens elements in the first lens unit, three lens elements in the second lens unit, one lens element in the third lens unit and four lens elements in the fourth lens unit. This lens system consists of lens elements in a number smaller than those of the lens elements used in the other conventional zoom lens systems.
There has recently been disclosed a zoom lens system consisting of a smaller number of lens elements, concretely nine lens elements. This zoom lens system was disclosed by Japanese Unexamined Published Patent Application No. 287810/63, has lighter weight and compacter design, a back focal length long for the focal length thereof, concretely an optical path length of back focal length 1.05 times as long as the geometrical mean of the focal length of the zoom lens system as a whole at the wide position thereof and the focal length of the zoom lens system as a whole at the tele position thereof, and permits various types of optical members to be inserted between the zoom lens system and the image surface thereof.
The above-described conventional zoom lens system has field angles of 45.5.degree. to 16.2.degree., a vari-focal ratio of 3 and an aperture ratio of F/1.4, but the zoom lens system comprises a first lens unit having a large diameter and thickness, and it cannot be said that the zoom lens system is sufficiently compact among the zoom lens systems of the similar type. Especially when the zoom lens system is combined with an electric focusing mechanism for performing automatic focusing, for example, the zoom lens system has a defect that it consumes power at a high rate and does not permit accelerating focusing speed.
Further, for designing a viewfinder optical system for single-lens reflex cameras, a relatively wide space is necessary between the zoom lens system and the image surface thereof for arranging a light splitting mirror for leading light into the viewfinder optical system. Especially when the light-splitting mirror is designed as a quick return mirror or a plate mirror, a wider space is required. The zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 287810/63 is insufficient in width of the space described above.